1. Field
The present invention relates to a mobile communication terminal that performs a cell search process and a cell search control program for the mobile communication terminal.
2. Description of the Related Art
Recently, portable mobile communication terminals having a wireless communication function have become widely used in accordance with prevalence of wireless communication lines. Cellular phones as examples of the mobile communication terminals having a wireless communication function include a voice communication function and a data communication function for communicating over a wireless communication line. The cellular phones are used for various purposes such as voice calls, transmission and reception of an E-mail, and website browsing. Recently, in accordance with the decrease in size and weight of cellular phones, the number of cases where a cellular phone continues to communicate while being moved a physically long distance is increased.
Generally, the cellular phones use a wireless communication system called a cellular communication system. In a cellular communication system, each base station sets up wireless communication lines with cellular phones located within a predetermined range (hereinafter, simply referred to as a cell) with the base station used as the center and communicates with the cellular phones. Cells are available by disposing a plurality of base stations, and cellular phones moving a long distance are moved across the cells.
Among the cellular communication methods, according to a communication method called OFDMA (Orthogonal Frequency Division Multiple Access) or CDMA (Code Division Multiple Access), when a cellular phone moves from a given cell to another cell, the cellular phone performs a so-called cell search process. By performing the cell search process, the cellular phone can set up synchronization of a wireless signal with a base station that provides the cell of the movement destination and detects the base station. Such a cell search process is generally performed in a case where a cellular phone is determined to have moved between cells. Determination on the movement of the cellular phone between cells is performed, for example, by regularly evaluating the value of reception signal power of the wireless signal that is received by an antenna of the terminal or the like and determining a case where the reception signal power is less than a predetermined level as movement between cells.
The cellular phone is capable of maintaining the connection of the wireless communication line even in cases where the cellular phone is moved across the cells by reselecting a cell or performing a hand-over process based on the result of the cell search process. Examples of such technique are disclosed in JP-A-2009-049661 and in JP-A-2001-186561.
However, according to the above-described cellular system, there are places, such as areas where there are many persons using cellular phones and traffic is concentrated, or places where there are many shielding objects and substantial radio wave interference nearby, in which a wireless communication line of sufficient signal quality cannot be provided even within a cell.
In order to provide a wireless communication line in such a place having a poor communication environment, a technique for disposing a small scaled base station (hereinafter, simply referred to as a femtocell base station) in a place having a poor communication environment and locally providing wireless communication lines is in practical use. Since the range in which a femtocell base station provides wireless communication lines is a small range on the order of several tens of meters, the range in which such wireless communication lines are provided is generally called a femtocell.
Since the above-described femtocell is disposed so as to supplement a place having a locally poor communication environment, there are many cases where the femtocell is disposed in a state being included in a range (hereinafter, simply referred to as a macrocell) in which a large scaled base station (hereinafter, simply referred to as a macrocell base station) provides wireless communication lines. In such a case, a cellular phone that is within the zone of the femtocell is simultaneously within the zone of the macrocell.
Generally, the number of wireless communication lines that can be provided by a base station is limited. Accordingly, in view of stability of wireless communication lines, it is preferable that the number of wireless communication lines established by one base station is small. However, the number of wireless communication lines established by a macrocell base station is vast based on the space of the cell area. Accordingly, in order to provide the wireless communication lines of a macrocell base station in a stable manner, a cellular phone that is simultaneously within the zone of a macrocell and the zone of a femtocell needs to establish a wireless communication line with the femtocell base station first.
While there is a request for allowing a cellular phone to be used in a femtocell first, in a conventional cellular phone as described above, determination on whether or not to perform a cell search process is made based on the evaluation of the signal quality of the wireless signal. Accordingly, in a case where the signal quality of the wireless signal transmitted from the macrocell base station is good, the cell search process is not performed, and thereby the femtocell cannot be detected. In such cases, there is a problem in that the wireless communication line cannot be set up with a femtocell base station even in a case where the cellular phone is simultaneously within the zone of the macrocell and the zone of the femtocell.
In addition, in a case where detection of such a femtocell is performed, as disclosed in the publication, JP-A-2009-049661, the cellular phone needs to perform the cell search process at a constant time interval. However, in a case where the cell search process is performed at the constant time interval, the cell search process is repeated at the constant time interval even in a case where the cellular phone is out of the zone of a femtocell. Accordingly, the processing load and the consumed current accompanied with the cell search process of the cellular phone are increased, and the continuous use time of the cellular phone may be decreased.